The conventional light emitting diode structure includes a growth substrate, an n-type semiconductor layer, a p-type semiconductor layer and a light-emitting layer between the two semiconductor layers. A reflecting layer used for reflecting the light from the light-emitting layer could be formed selectively in this structure. In order to improve at least one of the optical property, the electrical property, and the mechanical property in the light emitting diode, one adequately selected material would be used to substitute the growth substrate as a carrier to carry the structure except for the growth substrate, for example: metal or silicon substrate could be used to replace the sapphire substrate for growing nitride. The growth substrate could be removed by etching, polishing or laser-removing. However, the growth substrate could be also reserved entirely or partly and combined with the carrier. Besides, a transparent oxide could also be integrated in the light emitting diode structure to promote the current spreading.
The applicant disclosed one light-emitting device 100 with high light-emitting efficiency in TW Pat. No. I237903. As shown in FIG. 1, the light-emitting device 100 includes a sapphire substrate 110, a nitride buffer layer 120, an n-type nitride semiconductor stack 130, a nitride light-emitting layer of a multiple quantum-well structure 140, a p-type nitride semiconductor stack 150, and a transparent conductive oxide layer 160. Besides, a plurality of hexagonal-pyramid cavities 1501 formed on the surfaces where the p-type nitride semiconductor stack 150 facing the transparent conductive oxide layer 160. The inner surfaces of the hexagonal-pyramid cavities 1501 are easier to form the ohmic contact region with the transparent conductive oxide layer 160 wherein the material of the transparent conductive oxide layer 160 can be indium tin oxide (ITO), cadmium tin oxide (CTO), antimony tin oxide, indium zinc oxide, zinc aluminum oxide, or zinc tin oxide. Therefore, the forward voltage of the light-emitting device 100 keeps lower and the light-extracting efficiency is also improved by the hexagonal-pyramid cavities 1501.
ITO could be formed on the hexagonal-pyramid cavities 1501 of either or both of the semiconductor stacks by electron beam evaporation or sputtering. ITO with different forming method may show difference in the optical property, electrical property, or both, and the related reference could be referred to Taiwan Application No. 096111705, which is incorporated herein by reference by the same applicant. In FIG. 2, under a scanning electron microscope (SEM), the hexagonal-pyramid cavities 1501 are not fully filled with ITO particles by the electron beam evaporation and a lot of space formed between the particles. The space may confine the light to the light-emitting device and make the light being absorbed by the surrounding ITO gradually. Or there is the medium such as air with the smaller refractive index than that of ITO existing in the space, which may cause the total reflection at the boundary of the materials so the light could not leave the ITO layer and being absorbed by the surrounding ITO gradually.
In addition, C. H. Kuo et al. disclosed the relation between the ITO transmittance and light wavelength in the paper entitled “Nitride-based near-ultraviolet LEDs with an ITO transparent contact” (Materials Science and Engineering B, 2004). When the wavelength is smaller than 420 nm, the ITO transmittance tends to decrease rapidly, and is even lower than 70% when the wavelength is 350 nm. ITO has the transmittance higher than 80% in the blue light wavelength region, but the transmittance in UV or near-UV region is not good enough. Therefore, transparent oxide material such as ITO commonly used in the semiconductor light-emitting device has more space to improve for the performance in optics and electricity.